1. Field of the Invention
The present invention relates to an optical disk made of a plastic material and having pits on one surface thereof for storing data therein, which data can be regenerated by a light beam such as a laser beam.
2. Description of the Related Art
Recent remarkable developments of optical disks have caused these optical disks to supplant conventional recordable disks in the field of sound recording, and further in the field of electronic computers when used as CD-ROMs.
The conventional recordable disk is provided with a continuous spiral groove for storing data as sound or in the form of analogue signals, but an optical disk is provided with discontinuous pits (recesses) on one surface of the plastic body of the disk, to store data, such as sound, in the form of digital signals. To this end, the pits are formed in a predetermined pattern on one surface of the plastic optical disk, and a reflecting layer is formed by an aluminum evaporation process over the surface of the disk having the pits.
In addition, a protecting layer is formed over this reflecting layer. In the use of the optical disk, a laser beam is applied to the exposed surface of the plastic disk, opposite to the surface thereof containing the pits, so that the laser beam is transmitted through the plastic disk from the exposed surface to the surface having pits, and is reflected at the reflecting layer. The reflected beam is again transmitted through the plastic disk and is emitted from the exposed surface, and the output light is delivered to the regenerating device for reading the data stored in the optical disk, based on the intensity of the output light.
There are many severe requirements for a material for constituting an optical disk. For example, such a material must have such properties as a high transparency, to allow a light to be transmitted therethrough with a low loss, a low birefringence, a low hygroscopicity, to prevent a warping of the optical disk, a high resistance to heat, a high fluidity during a molding process, a good demolding property upon completion of a molding process, and a low contamination by foreign substances and impurities. To satisfy these requirements, an acrylic resin such as polymethyl methacrylate resin was developed as a material for the optical disk, and recently, a polycarbonate resin has been developed for an optical disk intended for use in a hot environment; for example, in an audio system in an automobile.
A problem arises if the plastic optical disk contains a large amount of foreign substances and impurities in that, when the laser beam is transmitted through the plastic disk, and the reflected light again transmitted through the plastic disk, the laser beam is absorbed by the foreign substances and impurities and the intensity of the output light becomes weak, and thus the signal to noise (S/N) ratio is lowered. Therefore, the development of optical disks has been concentrated on determining how to obtain a plastic material having a high transparency. It is, therefore, a conventionally immutable concept that a colorless transparent plastic material is used for the optical disk, and efforts have been made to establish a manufacturing process which precludes the entry of foreign substances and impurities into the optical disk material.
As the popularity of optical disks becomes greater, a demand has arisen for a colored optical disk, for example, a red or yellow disk, but a plastic material inherently having a good transparency and a desired color is not known, and to obtain an optical disk having a desired color, it is necessary to mix a coloring material into a transparent plastic material.
Nevertheless, a coloring material or a pigment for coloring is deemed to be an impurity, in view of the transmission of the laser beam, which coloring material absorbs the light to some extent, and accordingly, under the conventional concept of completely precluding foreign substances and impurities, it is not conceivable to use a coloring material at a region through which the laser beam is transmitted, and thus it is commonly considered that it is impossible to obtain a colored optical disk.